1. Field of the Invention
This invention relates to a silver halide photographic emulsion spectrally sensitized with at least two types of sensitizing dyes having a supersensitizing effect on each other and particularly, it relates to a silver halide photographic emulsion having an increased spectral sensitivity in the red-sensitive wavelength region.
2. Description of the Prior Art
It has been well known as one of the production techniques of photographic light-sensitive materials to broaden the light-sensitive wavelength region of a silver halide photographic emulsion toward the longer wavelength side, that is, subject the emulsion to spectral sensitization, by adding a certain cyanine dye to the emulsion.
It is also known that the spectral sensitivity is generally influenced by the chemical structure of the sensitizing dye added and various characteristics of the emulsion such as the halogen composition of the silver halides, the crystal habit, the crystal system, the silver ion concentration and the hydrogen ion concentration and also influenced by photographic additives present in the emulsion such as stabilizers, anti-fogging agents, coating aids, precipitating agents or color couplers.
For sensitization of a light-sensitive material in a predetermined spectral wavelength region, only one sensitizing dye is generally used. The combined use of sensitizing dyes often provides a spectral sensitivity lower than that obtained with the individual use of the sensitizing dyes. However, in special cases, a remarkable super-additive increase is obtained in spectral sensitivity when a sensitizing dye is used in combination with one or more other sensitizing dyes. This effect is known as supersensitization. However, a specific selection is required in combining the sensitizing dyes. Even a slight difference in the chemical structure significantly affects the supersensitizing effect, and therefore, it is difficult to predict which combination of sensitizing dyes will have a supersensitizing effect based only on the chemical structural formulas.
Moreover, the sensitizing effect on a particular emulsion can be generally varied by changes in the characteristics of the emulsion. For example, the sensitizing effect can be strengthened by increasing the silver ion concentration and/or decreasing the hydrogen ion concentration. Therefore, the sensitizing effect can be increased by immersing a film coated with a spectrally sensitized emulsion in water or an aqueous solution of ammonia. The above methods of changing the sensitivity of the spectrally sensitized emulsion by increasing the silver ion concentration and/or decreasing the hydrogen ion concentration are usually referred to as hypersensitization. The shelf life of hypersensitized emulsions is generally short.
When supersensitization is applied to a silver halide photographic emulsion, the sensitizing dyes used must not have adverse interactions with photographic additives other than the sensitizing dyes and stable photographic properties must be maintained during storage of the light-sensitive material.
Moreover, another requirement for the sensitizing dyes used is that "residual color" must not remain on the light-sensitive material due to the sensitizing dyes after processing. It is particularly required that no residual color is left after processing for a short time (usually several seconds to several tens of seconds) such as in rapid processing.
In order to obtain excellent color reproducibility in a color light-sensitive material, the red-sensitive layer preferably does not have a high sensitivity at too long a wavelength, for example, has a maximum sensitization at a wavelength longer than 660 nm and conversely, the red-sensitive layer preferably does not possess a sensitivity only in a too short wavelength region, for example, has a maximum sensitization at a wavelength shorter than 580 nm. Unfortunately, it is difficult with respect to spectral sensitization techniques to increase the sensitivity in the wavelength region where the maximum sensitization is situated below about 630 nm. Among all, it is particularly difficult to increase the sensitivity in the wavelength region ranging from 580 nm to 600 nm, and therefore, to solve this problem is one of the important subjects in the art.